1. Field of the Invention
The present invention relates to an improved electrical heat exchange device comprising a plurality of electrically conductive metal plates connected in series and through which a fluid to be heated is passed under pressure. Particularly, each conductive plate defines a long conductive path for the current, the length of which is predetermined depending on the various power voltage level that the device must be connected.
The electrical heat exchange device of the present invention was particularly developed to replace other electrical heating devices used to heat liquids or air forced therethrough. The electrical heat exchange device of the present invention is particularly useful in applications where a high power density is required, where fluid is required to be quickly heated, where the heating requires high temperatures, and where the heating device must utilize a minimum amount of space and be constructed at a minimum investment cost.
2. Description of Prior Art
Various types of electrical heat exchange devices are known such as open coiled type electrical heating elements, tubular type electrical heating elements, or screen type elements. However, these elements have various disadvantages. For example, open coiled type electrical heating elements have a volumetric power density factor of the order of 10 watts per cm.sup.3 with an air speed of 10 meters/second. Also, at this density, the increase in temperature of the fluid being heates is very limited in a reasonable heating volume.
Some porous electrical elements are known and these are usually fabricated from ceramic, carbon, etc. whichis relatively weak. Such heating elements have been found to be difficult and costly to build. One of the disadvantages of these elements is that it requires more pressure to move a fluid therethrough than other types of electrical heating elements for the same power transfer. In U.S. Pat. No. 3,344,247 there is disclosed an electrically powered fluid heater consisting of a plurality of metal screen sections positioned in spaced apart side-by-side relationship. The screen sections are held in a non-paralell curved fashion by supports and therefore the perforations in the screen sections are not aligned, and this creates deviations to air flow and results in pressure loss. Also, the resistance of the screen is very low as the conductive surface is the entire width and surface of the screen. Therefore the resistance, due to the nature of the conductive wire mesh, is very low because of the high electrical conductivity of the screen. The only parameter that may be modified to increase the electrical resistance for adaptation to standard voltage sources is to increase the number of screen sections. The disadvantage here is that the size of the housing may have to be made very large and impractical thereby decreasing the volumeric power density (Kw/meter.sup.3). Also, there is a larger fluid flow pressure loss due to the increased number of screen sections and finally the overall cost of the heater is increased. Therefore, such devices have only limited applications and cannot be used for high power density applications at the appropriate voltage.
As an example of the most recent prior art available, reference is made to the following publications: "Nouvelles Technologies", Journal Francais de l'Electrothermie No 1 A/s 1986, Pierre Faucher. "Porous Element Fluid Heating", IEEE Coloquium on Electroheat, London 1981, J. F. Pollock. "Processing of Liquids Using Porous Element Heaters", 10th Congress on Electroheat, Stockholm 1984, R. G. P. Kusay.